104 FOSSIL REPTILIA OF THE 



The inverted arch 1 supporting the ' pectorals ' is detached from the occiput, as in the 

 Plagiostomes ; that supporting the ' ventrals ' is also detached from the ' sacrum,' but 

 retains the position beneath the vertebrae, which, when coalesced, receive that anthro- 

 potomical name. The hinder arch 1 has gained a structure determinative of the homology 

 with the hae mo vertebral elements called ' pelvis,' and the limbs so supported are called 

 ' pelvic' 



The pectoral arch (Plate XXVIII, fig. 4) consists of a pair of scapulae (51), a pair of 

 coracoids (52), a pair of clavicles (58), and an episternum (46). In some specimens there 

 appears a trace of a pair of precoracoids. 



The correspondence with the same arch in Omithoryhnchus was pointed out and 

 figured by Clift. 2 I have not seen an Ichthyosaurus in which the clavicles were 

 confluent mesially as a single bony arch, resembling- the Avian ' furculum ; ' but such 

 confluence does take place in the full-grown or aged Monotremes. No sternebers succeed 

 the episternum in Ichthyosaurus as they do in Ornithorhynclius. 



The episternum is small ; each clavicle exceeds the length of the anterior transverse 

 ray ; the medial longitudinal ray or stem does not exceed the transverse portion in extent. 

 The clavicles are powerful bones, pointed at each end, overlying the transverse rays of 

 the episternum, and continued along the anterior border of the scapulae towards or near 

 to the ' base ' or free extremity of those bones ; the joints are rough or sutural. The 

 scapulae are oblong, subcompressecl, truncate at the free or basal end, thickened and 

 broadened at the opposite or articular end for the two joint-surfaces of the coracoid 

 and humerus (53) respectively. 



The two pairs of limbs (ib., fig. l,s,p) have been found in every sufficiently preserved 

 skeleton, and where such fins have been lost their supporting arches or some elements 

 thereof have usually indicated their existence. Of these limbs the anterior or pectoral ( 5 ) 

 surpass in size, but in different degrees according to the species, the posterior or pelvic (p) 

 pair. They appear to be most nearly equal in size in the skeleton, in part restored, of the 

 Ichthyosaurus platyodon (PI. XXXI, fig. 1), but confirmatory evidence of the degree of 

 difference is desirable in regard to this species. The pelvic pair is the smallest relatively 

 in Ichthyosaurus latimanvs, Ich. communis, and Ich. breviceps, but the inferiority is 

 nearly the same in Ich. intermedins (PI. XXX, fig. I). 



In all the species the digits are supported by flattened, subquadrate, hexangular, 

 pentangular, transversely quadrate, or rounded phalanges, exceeding in number in each 

 digit that known in any other Reptile, and recalling the many -jointed rays of the pectorals 

 and ventrals of Fishes. 



The shorter-snouted species have the greater number of digits, with more and smaller 

 pnalanges ; as the jaws proportionately elongate the number of digits decrease, and their 

 phalanges become relatively larger and fewer. 



1 The definition of * girdle ' in our Dictionaries is inapplicable to these parts of the skeleton. 



2 ' Philos. Trans.,' mdcccxviii, p. 32, pi. ii. 



